1. Field of the Invention
The present invention relates to a process for producing acicular .gamma.-FeOOH particles useful for starting material, from which magnetic particles suitable for magnetic recording can be produced.
2. Description of the Related Art
There have been most widely used heretofore iron oxide based magnetic particles and metallic iron based ones to produce magnetic recording mediums such as audio tapes, video tapes, magnetic disks, and magnetic cards. Generally, known processes for producing these magnetic particles include a process where a solution of any one of various iron salts such as iron sulfate and iron chloride is neutralized with alkali and then oxidized to produce hydrous iron oxides such as acicular .alpha.-FeOOH (goethite), and .gamma.-FeOOH (lepidocrocite) as precursors which are sequentially subjected to firing (dehydration, tempering), reduction, oxidation and the like to produce acicular .gamma.-Fe.sub.2 O.sub.3 (maghemite), FeO.sub.x (1.5&lt;x.ltoreq.1.33: berthollide), Fe.sub.3 O.sub.4 (magnetite); and a process where the iron oxides as described above are further subjected to a modification treatment with cobalt to produce Co--.gamma.-Fe.sub.2 O.sub.3, Co--FeO.sub.x, or Co--Fe.sub.3 O.sub.4. Furthermore, metallic magnetic particles can be produced by performing further reduction in the process as described above without terminating the reduction at the stage of Fe.sub.3 O.sub.4 (magnetite).
Recently there has been a need for magnetic particles having more superior magnetic characteristics as magnetic recording mediums have been highly upgraded. In order to obtain more superior magnetic characteristics, improvement of magnetic characteristics such as coercive force, saturation magnetization and orientabilty as well as micronization of magnetic particles must be achieved. For the production of such magnetic particles having improved characteristics, it is important to improve the precursors for various properties such as particle size, distribution of particle size and crystallinity as well as to conduct the heat-treatment steps such as firing, reduction and oxidation as described above without causing any sintering of the precursor particles and any deformation thereof to produce magnetic particles retaining high acicularity.
The precursors include typically .alpha.-FeOOH (goethite) and .gamma.-FeOOH (lepidocrocite), and the .gamma.-FeOOH is characterized by scarcely generating twinned crystals in the precipitating course in contrast to .alpha.-FeOOH. Therefore, the ultimate magnetic particles produced from the precursor .gamma.-FeOOH are said to be linear acicular particles having no branched structure which have a higher dispersibility in synthetic resin mediums in the course of processing the particles into magnetic tapes, an excellent orientability and a higher coercive force to facilitate the production of much better products. On the other hand, however, .gamma.-FeOOH is more thermally unstable than .alpha.-FeOOH and hence it is difficult for the particles of .gamma.-FeOOH to retain their acicularity during the heat-treatments such as firing, reduction and oxidation, which has obstructed taking the advantages of .gamma.-FeOOH until now. Thus, .alpha.-FeOOH has been rather employed as precursor in numerous instances.
Some attempts have been made heretofore to employ the treatment with a phosphorus compound and the like in the course of the production of .gamma.-FeOOH or magnetic particles from the precursor, .gamma.-FeOOH as proposed in, for example, 1) Japanese Patent Publication, No. 43-2214; 2) Japanese Patent KOKAI (Laid-open) No. 61-256705; 3) Japanese Patent KOKAI (Laid-open) No. 56-41835; and 4) Japanese Patent KOKAI (Laid-open) No. 63-69714. The publication 1) discloses a process for producing .gamma.-FeOOH at a high space velocity and a high yield by adding a water soluble phosphorus compound or an arsenic compound before or after the nucleation reaction of .gamma.-FeOOH allowing the growth reaction to be effected at a higher temperature than that used in the previous methods, thereby achieving a higher rate of growth of .gamma.-FeOOH nuclei to produce a precursor suitable primarily for pigment iron oxide particles. The patent KOKAI 2) discloses a process for producing acicular metallic magnetic particles having a high coercive force and a low noise by conducting the nucleation reaction of .gamma.-FeOOH in the presence of a phosphorus compound, effecting the growth of the resultant nuclei to produce .gamma.-FeOOH particles, then performing a specific surface treatment, dehydrating, and then reducing the resulting particles under heat. The patent KOKAI 3) discloses a process for producing acicular metallic iron particles by coating a phosphorus compound and an organic carboxyl acid on the surfaces of .gamma.-FeOOH particles or a mixture of .gamma.-FeOOH and .alpha.-FeOOH particles, and heat-reducing the coated particles under specific conditions. The patent KOKAI 4) discloses a process for producing magnetic iron oxide particles by treating hydrous iron oxide particles such as .gamma.-FeOOH particles with a phosphorus compound in vapor phase during firing, and then conducting the reduction and oxidation of the treated particles.
In these processes, however, as in the patent 1) and 2), the addition of phosphorus compounds in the course of the nucleation reaction or the initial crystal growth reaction of .gamma.-FeOOH particles has a great influence on the configuration of the .gamma.-FeOOH particles produced and a slight variation in the amount of the phosphorus compounds to be added may result in a great variation in size and particle configuration of the .gamma.-FeOOH particles produced to adversely affect the acicularity of the particles. Particularly, the process of the patent 1) is characterized by the production of cubic particles suitable for the precursor materials, from which iron oxide particles for pigments can be produced, and it does achieve little effect to preserve the acicularity of the particles as antisintering agent in the course of production of magnetic iron oxide particles or magnetic metallic iron particles through the heat-treatment of the produced .gamma.-FeOOH as described above.
The process of coating the phosphorus compounds on the surfaces of the produced .gamma.-FeOOH particles as proposed in the patent 3) has a very low coating efficiency of the phosphorus compounds onto the surfaces of the .gamma.-FeOOH particles, and it is difficult to coat the phosphorus compounds in a controlled amount because the amount is influenced by the size of the .gamma.-FeOOH particles, which makes it difficult to consistently control the magnetic characteristics of the magnetic metallic iron particles, which can be produced by the heat-treatment, with an actually coated amount being too low or too high, though the phosphorus compounds are appreciated to have an effect as antisintering agent.
The process of treating the surfaces of the produced .gamma.-FeOOH particles with the phosphorus compounds and the like in vapor phase as propsed in the patent 4) can not be said advantageous in industry in view of the fact that the phosphorus compounds to be used are very toxic such as PH.sub.3, P.sub.2 H.sub.4 and the like as well as of facilities and operations to be employed.